While stereospecific opiate receptor assays have been employed to study the regional and subcellular location of the opiate receptor(s) in the brain, little is known about its molecular characteristics. In order to investigate the opiate receptor in greater detail, the preparation of high affinity photoreactive labels is proposed. Candidate labels derived from levorphanol and met5-D-ala2-enkephalin will be tested by binding assay and bioassay methods to evaluate potency and agonist/antagonist properties. Superior candidate labels will be converted to tritiated analogs of high specific activity (greater than or equal to 10 Ci/mmole) and the stereospecific binding of these tritiated analogs will be verified. Synaptosomal membranes will be treated with the tritiated opiate receptor photoaffinity labels in the presence of either unlabeled levorphanol or dextrorphan. After collection of the membranes on glass fiber filters and rapid filter washing, the filters will be photolyzed to covalently lock the photoreactive labels in place. This procedure should prevent any redistribution of stereospecific binding. The labeled membranes will be solubilized by treating the photolyzed filters with sodium dodecylsulfate (SDS). After dialysis to remove unbound radioactivity, SDS extracts from the synaptosomal membranes labeled in the presence of either levorphanol or dextrorphan will be subjected to SDS-polyacrylamide gel electrophoresis. The resulting gels will be sliced into sections and the distribution of radioactivity determined by liquid scintillation counting. By subtracting the cpm of slices of levorphanol gels from the cpm of the corresponding slices of dextrorphan gels, stereospecific receptor binding will be determined. From these data, the apparent molecular weight(s) of the receptors can be ascertained.